In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the surface of the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is then cleaned to remove therefrom any remaining coagulated colloid.
The rubber squeegee which is used to remove any remaining non-coagulated colloid from the surface of the positive electrode comprises an elongated blade member having a generally triangular cross-section with a longitudinal axis extending parallel to the rotation axis of the positive electrode and a transverse axis which inclined in a direction opposite to the direction of rotation of the electrode at an angle of about 50 relative to the radius thereof. Applicant has observed that it is necessary to apply with such a squeegee a linear loading of about 17 N/cm onto the surface of the electrode in order to completely remove therefrom all non-coagulated colloid. Such a high linear loading, however, causes an abrasion of the dots of colored, coagulated colloid formed on the surface of the electrode so that the coagulated colloid is no longer representative of the desired image. The same problem has also been encountered with the squeegee device described in Applicant's copending U.S. application Ser. No. 08/665,458.